1. Field of Invention
The present invention relates to a line control unit for teleprinters, comprising a circuit for generating switching signals, a pair of transistors connected to sources of positive and negative voltage, respectively, and furthermore interconnected at a point common to the line to supply selectively on the line a positive and negative current, respectively, and a pair of isolating circuits disposed between the transistors and the generating circuit so that there is no electrical connection between the line and the generating circuit.
2. Description of the Prior Art
As is known, one of the main functions performed by a line control unit is that of effecting electrical isolation between the teleprinter apparatus and the telegraphic line. In known line control units, this function has been performed by decoupling relays driven by the signals issuing from the teleprinter and generating the line signals. In more modern line control units, the relays have been replaced by optoelectronic couplers which are formed by a photoemissive diode and photosensitive transistor coupled optically to the diode. The photosensitive transistor is connected to an amplifier which drives a line transistor. More particularly, in a double-current line, the line current is modulated from a negative value to a positive value in correspondence with each information bit, for example from -25mA to +25mA.